Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Marketable securities sold during period, fair value	$ 127,600	$ 9,500
Marketable securities sold during period, amortized cost basis	127,700	9,500
Net realized losses	100
Proceeds from sale of marketable securities	127,000	9,500
Reclassification unrealized net holding losses	125
Fair value of marketable securities in unrealized loss position	2,500	40,400
Aggregate unrealized losses for securities in unrealized loss position	3	57
Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Fair value of marketable securities in unrealized loss position		31,800
Aggregate unrealized losses for securities in unrealized loss position	0	48
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Fair value of marketable securities in unrealized loss position		6,600
Aggregate unrealized losses for securities in unrealized loss position	$ 3	9
Bank certificate of deposits
Schedule of Available-for-sale Securities [Line Items]
Fair value of marketable securities in unrealized loss position		1,000
Aggregate unrealized losses for securities in unrealized loss position		0
U.S. Treasury securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Fair value of marketable securities in unrealized loss position		1,000
Aggregate unrealized losses for securities in unrealized loss position		$ 0